I noticed this behaviour after the latest FW upgrade. Also I searched the forum and found out that others have had the same problem too. None of those threads have helped me though. I've tried disabling all unneeded and also "needed" services, just to see what keeps the drive from not going to standby.

For this I downloaded smart_spindown script and debugged dmesg messages. The drive is quiet (according to the script) for all the time.

Drive(s) spun down after 270 seconds (with 240 seconds of inactivity), total spin down count: 2.
spindowns: 2, time up/down: 330/0, backoff 1600, down for 0 (avg 0).
A drive spun up after 0 seconds. Total time up/down: 360/0 (avg sleep time 0)

So does it actually even spin down (not next to the device at the moment)?

Right after this it says the disk is quiet for the next xxx seconds and then it "spins down" again, and "spins up after 0 seconds".

This is what dmesg tells me (cleared with dmesg -c right after starting the script by hand):

Just a quick chase up…I tried smart_spindown script while I was next to the MyBook, and it started to spin down for a split second, but immediately spun back up. So something is blocking it from spinning down.

I also tried without the network cable for ~30 min (that time I had the default "standby setting" on), but it never went to standby.

I don't know what the problem is with your MBWE, but I do know that it took me a few days to get my Smart_Spindown script to work properly. Here are the extra things i had to do in addition to all the steps in the tutorial. It takes a lot of trial and error to isolate the problem scripts.

It's weird, I don't have most of the stuff your link shows, like kill_idle_fcgi and the php settings in featurepack manager. It's probably because I have the white led version (different featurepacks?)

Ok I guess it was either openvpn.conf with the whatever change the link you posted contained, or it might've been NFS or AFP, I didn't even remember that I had those on (even though I don't need them). So I disabled NFS and AFP and noticed that my HD was on standby this morning. So consider it working now :)